Bi-stable reflective displays based on PDLC are known. A polymer dispersed liquid crystal (PDLC) system contains droplets of liquid crystal material dispersed in a polymer matrix. Such systems are known in the art and have been disclosed by Doane et al. (Applied Physics Letters 48, 269 (1986)), West et al. (Applied Physics Letters 63, 1471(1993)) and by Stephenson (U.S. Pat. No. 6,359,673). The PDLC may be used to create passive matrix displays on flexible substrates. See also U.S. Pat. No. 6,061,107 to Yang et al., incorporated by reference. Chiral-nematic liquid-crystals, also referred to as cholesteric liquid crystals, have the capacity of maintaining (in a stable state) one of a plurality of given states in the absence of an electric field.
U.S. Pat. No. 5,251,048 to Doane et al. and West et al. disclose a PDLC based bi-stable reflective display. The device comprises droplets of chiral nematic liquid crystal (CLC) in a polymer binder coated between two transparent electrodes. The CLC material can be switched between a reflecting planar state and a weakly scattering focal conic state by application of voltage pulses of different magnitudes. The planar and focal conic states are both stable at zero applied field. West et al. note a change in shape of the reflection spectrum of the CLC material when it is confined to the geometry of a dispersed droplet. They note that the reflectivity changes much less with observation angle compared to the original chiral nematic mixture. However, they disclose the spectrum of only a single CLC material with peak reflectivity of 564 nm and do not teach methods for obtaining a broadband reflecting display having a more neutral appearance. Furthermore, although they note that the domain structure of the dispersed system in the focal conic state scatters light uniformly over the visible portion of the spectrum with the back scattered intensity gradually increasing at lower wavelengths, they do not specifically teach methods to improve contrast of the display.
Stephenson in U.S. Pat. No. 6,359,673 discloses a PDLC based bi-stable reflective display. A neutral appearance in the reflective state is obtained by combining the reflections of droplets of CLC material doped to give reflections either in the blue, green and red part of the spectrum or the blue and yellow part of the spectrum. Stephenson does mention that the polymer-dispersed liquid crystals have good off-axis reflectivity but does not teach how this property is to be used to create more neutral looking displays with improved viewing angle dependence. Stephenson teaches that domains or droplets of liquid crystal in the light-modulating layer are smaller than the thickness of the layer so that multiple domains overlap. Stephenson further teaches that the contrast of the display may be improved by combining multiple overlapping domains comprising right handed and left handed chiral-nematics. However, Stephenson does not mention anything about the effect of this on the back-scattered intensity in the focal conic state.
T. Kakinuma et al., in “Black and White Photo-addressable Electronic Paper using Encapsulated Cholesteric Liquid Crystal and Organic Photoconductor,” IDW '02, page 1345 to 1348 discloses a liquid crystal display using red (pink) and green capsules to create broadband. However, Applicants have determined that using an imaging layer having substantially more than a monolayer of domains (as shown by the enlarged portion of FIG. 1 results in relatively low contrast ratios.
U.S. Pat. No. 5,875,012 A to Yang et al. discloses a broadband reflective display having improved reflectivity as well as a lack of haze or opaqueness when viewed from various viewing angles, a reduced drive voltage compared to conventional devices, and an improved peak photopic reflectance. The broadband reflective display includes a plurality of pairs of substrates, a plurality of groups of alternating liquid crystal and polymer layers formed between each of the pairs of substrates, each of the groups of liquid crystal and polymer layers being reflective of different wavelengths of light.
U.S. Pat. No. 5,847,798 discloses (particularly in FIG. 7) a liquid crystal cell having multiple stable reflecting states between a colored reflecting state and a light scattering state in order to allow for a substantially white background. Under room light conditions, where light is incident on the cell from all directions, the light reflected from different domains has different colors because the incident angles θ in different domains are different. As such, the light observed by a human eye is an average of the reflection bands centered at different wavelengths and has a substantially white appearance.
U.S. Pat. No. 6,034,752 to Khan discloses a liquid crystal device in which the liquid crystal material has a pitch length effective to reflect radiation having both the visible and the infrared ranges of the spectrum, either in a single region (single cell) between opposing substrates or in separate regions, for example, in stacked regions in which a first cell reflects red light and a second cell reflects blue light and a third cell reflects green light. Example 2 discloses a composition for a single cell display that reflects both visible and infrared radiation. The Examples do not employ domains of liquid crystal in a polymer matrix and the imaging layers are not coated on a flexible substrate.
Chiral-nematic liquid crystals reflect a portion of the visible spectrum when in a reflective state. It is preferable that the reflective state has neutral color balance. It would be useful to provide chiral-nematic displays exhibiting neutral density in the reflective state. It would be useful for such displays to be fabricated using simple, low-cost processes.